Over the past few years, effort in the prior art has been expanded to develop a reluctance machine with high efficiency and high torque production capability relative to physical size. A reluctance machine is a type of magnetic device, e.g., a motor, where magnetic poles are induced in a rotating non-magnetic member (i.e., a rotor) by at least one winding in a stationary member (i.e., a stator). The rotor is typically provided with a plurality of salient (i.e., outwardly projecting) poles. The poles are induced by applying electrical current to the winding.
A reluctance machine can be constructed in the form of at least a synchronous reluctance machine, a variable reluctance machine, or a switched reluctance machine. A synchronous reluctance machine is configured to have a rotating field in the stator windings. A magnetomotive force (MMF) is thereby produced which acts upon the rotor resulting in the rotor being rotated at a synchronous speed which is substantially equal to the speed of the rotating field in the stator.
Reluctance machines can be constructed to have distributed windings or concentrated windings. In a reluctance machine with distributed windings, the windings are distributed substantially around the entire stator. A winding can be as simple as a single coil, although a single-coil winding is atypical, or generally, the winding can include multiple coils in series/parallel, where the coils are different sizes in that they have different numbers of turns and span different portions of the stator. Generally, the pattern of a distributed winding can be complicated. In either case (i.e., distributed or concentrated windings), the windings can be configured to be coupled to a voltage source with single, two, or multiple phases. Each winding is associated with one phase. For example, using a delta and wye connectivity, as is known to a person having ordinary skill in the art, three-phase power can be coupled to three windings (distributed or concentrated).
Exemplary prior art reluctance machines with distributed windings (FIG. 9A) and concentrated windings (FIG. 9B) are depicted in FIGS. 9A and 9B. A cross-sectional view of a typical reluctance machine configuration with distributed windings is shown in FIG. 9A. The reluctance machine 10 includes a stator assembly 12 and a rotor assembly 14. The stator assembly 12 includes a stator body 20 (also referred to as housing) with a plurality of stator teeth 22 coupled thereto. Windings (not shown) are provided around and about the stator teeth 22 in a distributed fashion. The rotor assembly 14 is centrally mounted within the stator assembly 12. The rotor assembly 14 includes a plurality of protruded poles 30 each having a pole face 32 that is substantially parallel with an interior surface defined by adjacent stator teeth 22. The rotor assembly 14 further includes a rotor core 26 and a shaft 24 centrally located about the rotor core 26. The reluctance machine 10 depicted in FIG. 9A includes a large number of stator teeth where many teeth are associated with a winding (not shown), and the associated phase. In the exemplary embodiment of the prior art depicted in FIG. 9A, the conventional reluctance machine may include three windings, each associated with a phase, where the windings are distributed around the teeth 22 (where a large number of teeth 22 are provided around the stator body). The rotor assembly 14, as depicted in FIG. 9A, includes four protruded poles 30, although it may include less or more poles.
Referring to FIG. 9B, another exemplary embodiment of a stator assembly 40 is depicted. Similar to the stator assembly 12, the stator assembly 40 includes a stator body 42 and a plurality of stator teeth 44. Between each set of stator teeth 44 are cavities formed within the stator body 42 to accommodate a plurality of concentrated windings 46. In the embodiment of FIG. 9B, the windings 46 are adapted to be powered by a three-phase power configuration, as identified by phase connectivity P1, P2, and P3. Application of electrical currents to the windings 46 generate a rotating field which causes a rotor assembly (e.g., the rotor assembly 14 of FIG. 9A) to rotate within the space defined by the stator assembly 40. The rotating field results in an output torque provided from the reluctance machine (e.g., the reluctance machine 10 of FIG. 9A).
Referring back to FIG. 9A, it should be noted that spacing (i.e., the air gap) between the pole face 32 of each pole 30 and the adjacent stator teeth 22 remains substantially constant. The consistency in the air gap results in the same torque output capability in either a clockwise 38 or a counter clockwise 36 rotational direction. In particular, as a leading edge 34b of a rotor pole 30 approaches the next stator tooth 22 when the rotor assembly 14 is rotating in the direction depicted by arrow 38 (i.e., clockwise directions), the output torque remains substantially the same as if a trailing edge 34a of a rotor pole 30 approaches the next stator tooth 22 when the rotor assembly 14 is rotating in the direction depicted by arrow 36 (i.e., counter clockwise) corresponding with a reversal of the rotational direction of the stator MMF.
Attempts to improve the performance of the synchronous reluctance machines are typically associated with design of the rotor assembly 14 of the reluctance machine 10 such that it will result in improved performance. One category of performance is torque density which is the amount of torque that is generated relative to the physical size or mass of the machine for a given amount of loss. The rotor assembly 14 depicted in FIG. 9A, although simple and can be manufactured at a relatively low cost, has relatively poor performance in terms of torque density, since flux density (i.e., the MMF resulting in output torque) varies considerably over the pole faces 32 of the pole 30, as discussed further below. Therefore the spatial region of high flux density is limited (which is a function of position within the rotor assembly 14), if a high degree of saturation (which leads to high loss) is avoided.
Several rotor designs can be found in the prior art. These include rotors with slits, rotors with segmentation, rotors with axial lamination, and rotors with transverse lamination. These rotor designs are generally directed at improving torque characteristics of the machine, however, the goal remains in developing a reluctance machine with further improved torque density for a given loss level over the rotor designs provided in the prior art. Furthermore, a rotor design that can improve the uniformity of the field over the rotor pole to improve torque ripple remains elusive.
Therefore, there is a need for a reluctance machine that improves output torque density based on the relationship between the rotor shape and the stator as well as providing a substantially uniform and maximized flux density over the face of the rotor pole (without a high degree of saturation and loss) thereby substantially maximizing toque density while substantially minimizing torque ripple.